A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus may be of the transmissive type, where radiation is passed through a mask to generate the pattern, or of the reflective type, where radiation is reflected from the mask to generate the pattern. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
It is often desirable to produce a lithographic apparatus capable of resolving features as small and close together as possible. Furthermore, it is often desirable for the maximum resolution available to be uniform across the width of the target portion. The size of the smallest resolvable feature is called the critical dimension (CD). A number of parameters affect the CD at the substrate. One of these is the transmission or reflectance of the mask. If the transmission or reflectance across the mask is not uniform, then the CD across the target portion will not be uniform. Non-uniformity of the transmission or reflectance of the mask may be caused by non-uniformity in the transmission or reflectance of the mask substrate or a variation in the size of features on the mask, or a combination of both. The size of features on the mask leading to the smallest resolvable feature at the substrate gives rise to a critical dimension of the mask. It will be appreciated that, due to the magnification of the projection system, the value of the critical dimension of the mask and the critical dimension (CD) of the substrate may not be the same.
Furthermore, there may be variations in the intensity of the projection beam across the target portion, and this will also lead to a non-uniform CD distribution across the target portion.
It is possible to correct for non-uniformity of intensity in the projection beam and in the transmission or reflectance of the mask, if this non-uniformity is known, by varying the illumination dose. It is therefore desirable to measure the non-uniformities accurately. It is possible to measure the homogeneity of the spatial distribution of the projection beam of a transmissive system simply by removing the mask and utilising a spot sensor at the wafer level. However, this does not take into account the variations introduced by the mask.
It is not practical simply to remove a mask from a reflective system, although a production mask can be replaced by a “blank” mask (i.e. a mask without a pattern) to reflect the beam towards the projection system in an effort to determine the non-uniformity of the projection beam. However, this enables the measurement at wafer level of the sum of the projection beam uniformity as provided by the illumination systems and the reflection coefficient of the blank mask. The problem with this approach is that the reflection coefficient uniformity of a blank mask is generally not the same as that of a production mask, so a correction based on the uniformity of a blank mask will be suboptimal for the production mask.
It is possible to determine the uniformity of the projection beam at substrate level with a production mask in place by exposing a test wafer using a production mask in the usual way. The test wafer can then be analysed using a scanning electron microscope (SEM) to determine the CD variation across it. The CD variation can then be corrected for by varying the radiation dose. However, this procedure involves a long cycle time and generally requires off-line tooling and related data logistics. It would be desirable to measure the uniformity of the projection beam with a production mask in place, without going through the complicated procedure of exposing and analysing a test wafer.